Viscous Fluid Coupling

ABSTRACT

A viscous fluid coupling includes a housing rotatably supported to a drive shaft of an engine. An operation plate set in the housing divides a space of the housing into a reservoir and an operation chamber. The operation plate has a communication hole communicating the reservoir and the operation chamber. A rotor fixed to the drive shaft is disposed in the operation chamber. A valve mechanism installed to the operation plate to close and open the communication hole according to ambient temperature of the housing. A driven wheel fixed to the housing is located between the operation plate and the rotor. A torque transmitting section includes a first annular projection concentrically formed on the driven wheel and a plurality of second annular projections concentrically formed on the rotor. The first annular projections are overlappedly adjacent to the second annular projections so as to establish fluid coupling therebetween through the viscous fluid.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of United States Patent Application No. 09/864,214filed May 25, 2001 now U.S. Pat. No. 6,474,458 which is a continuationof United States patent application Ser. No. 09/288,716 filed on Apr. 9,1999 in the name of Hirofumi KATOH et al., now U.S. Pat. No. 6,305,519B1 issued on Oct. 23, 2001 in the name of KATOH et al.

BACKGROUND OF THE INVENTION

The present invention relates to a viscous fluid coupling adapted todrive an accessory device, such as a cooling fan, of an internalcombustion engine.

Japanese Utility Model Provisional Publications Nos. 57-204491 and3-77825 disclose a viscous fluid coupling adapted to a cooling fan foran internal combustion engine. As shown in FIGS. 21 and 22, thisconventional viscous fluid coupling 100 includes a housing 104relatively rotatable with a drive shaft 102, an operation plate 108dividing a space in the housing 104 into a reservoir 105 and anoperation chamber 106, a rotor 109 disposed in the operation chamber 106and fixed to the drive shaft 102 and a valve mechanism 111 disposed atan outer peripheral portion of the rotor 109. The housing 104 isconstituted by a housing body member 121 supported by the drive shaft102 and a cover member 122 fixed to the housing body member 121. Thecover member 122 includes a circular recess portion 123 for defining theoperation chamber 105 and a ring-shaped flange portion 125 having areturn passage 124. The operation plate 108 is made of metal and formedinto a disc shape. The operation plate 108 is connected to the flangeportion 124 of the cover member 122 by means of caulking so as to closean opening of the recess portion 123 of the cover member 122. A torquetransmitting section 110 is constituted by a plurality of annularprojections 127 concentrically formed on the cover member 122 and aplurality of annular projections 128 concentrically formed on the rotor109 so that the annular projections 127 and 128 are overlapped with eachother.

When the ambient temperature around the housing is low, thecommunication hole 107 formed on the operation plate 108 is closed bythe valve mechanism 111 to stop the flow of the viscous fluid from thereservoir 105 to the operation chamber 106. Therefore, the torquetransmission amount from the rotor 109 to the housing 104 is lowered tostop or rotate a cooling fan in low speed. When the ambient temperaturearound the housing is high, the communicated hole is opened by theoperation of the valve mechanism 111 to allow the viscous fluid to flowfrom the reservoir 105 to the operation chamber 106. Therefore, thetorque transmission amount from the rotor 109 to the housing 104 isincreased to rotate the cooling fan at high speed.

However, this conventional viscous fluid coupling 101 encountersdrawbacks. For example, since the torque transmitting section 110 isformed by locating the annular projections 127 and the annularprojections 128, adjacent to each other it is necessary to locate thetorque transmitting section 110 on an outer peripheral portion of thecover member 122. That is, because the operation plate 108 is installedat a central portion of the cover member 122, it is impossible to locatethe annular projections 127 at an inner position corresponding to theoperation plate 108.

Further, since the torque transmitting section 110 is formed at an outerside as compared with an inner surface defining the reservoir 105, thatit, since the reservoir 105 is located at a height which is(gravitationally) lower than that of the torque transmitting section110, the torque transmitting section 110 becomes dipped in (viz, becomesat least partially immersed in or coated with) the viscous fluid whenthe engine stops. This dipping of the torque transmitting section 110generates a dragging-rotation phenomenon of the cooling fan when theengine is started. Although Japanese Utility Model ProvisionalPublications Nos. 59-128933 and 1-83925 have proposed anotherconventional viscous fluid coupling arranged to prevent such a fandragging-rotation phenomenon, this conventional viscous fluid couplinghas generated another problem that the utility of the viscous fluid isdegraded.

SUMMARY OF THE PRESENT INVENTION

A viscous fluid coupling according to the present invention is connectedto an engine. The viscous fluid coupling comprises a drive shaftconnected to a rotation shaft of the engine. A housing is rotatablysupported to the drive shaft. An operation plate is set in the housingso as to divide a space defined by the housing into a reservoir and anoperation chamber. The operation plate has a communication holecommunicating the reservoir and the operation chamber. Viscous fluid isstored in the reservoir and the operation chamber. A rotor is disposedin the operation chamber and is fixed to the drive shaft. A valvemechanism closes and opens the communication hole according to ambienttemperature of the housing to control a flow rate of the viscous fluidfrom the reservoir to the operation chamber. A driven wheel is fixed tothe housing so as to be located between the operation plate and therotor. A torque transmitting section includes a plurality of firstannular projections concentrically formed on an outer peripheral portionof the driven wheel and a plurality of second annular projectionsconcentrically formed on an outer peripheral portion of the rotor. Thefirst annular projections are overlappedly adjacent to the secondannular projections so as to be fluidly coupled with each other throughthe viscous fluid.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings, like reference numerals denote like parts and elementsthroughout all figures, in which:

FIG. 1 is a cross-sectional view of a viscous fluid coupling of a firstembodiment according to the present invention;

FIG. 2 is an enlarged cross-sectional view of an essential portion ofFIG. 1;

FIG. 3 is a plan view of an operation plate employed in the firstembodiment of FIG. 1;

FIG. 4 is a plan view showing a first surface of a driven wheel employedin the first embodiment;

FIG. 5 is a plan view showing a second surface of the driven wheel;

FIG. 6 is a cross-sectional view taken in the direction of section lineVI—VI of FIG. 5;

FIG. 7 is a plan view showing a modification of the driven wheel of thefirst embodiment;

FIG. 8 is a plan view of the rotor of the first embodiment.

FIG. 9 is a cross-sectional view taken in the direction of the arrowsIX—IX of FIG. 8;

FIG. 10 is a cross-sectional view of first annular projections of therotor employed in a second embodiment;

FIG. 11 is a cross-sectional view of the viscous fluid coupling of thethird embodiment according to the present invention;

FIG. 12 is an enlarged cross-sectional view of an essential portion ofFIG. 11;

FIG. 13 is an enlarged cross-sectional view of an essential portion ofthe fluid coupling of a fourth embodiment according to the presentinvention;

FIG. 14 is an enlarged cross-sectional view of the first and secondannular projections of the fourth embodiment;

FIG. 15 is an enlarged essential portion of the fluid coupling of afifth embodiment according to the present invention;

FIG. 16 is an enlarged cross-sectional view of the first and secondannular projections of the fifth embodiment;

FIG. 17 is an enlarged cross-sectional view of an essential portion of amodification of the fifth embodiment according to the present invention;

FIG. 18 is an enlarged cross-sectional view of an essential portion ofanother modification of the fifth embodiment according to the presentinvention;

FIG. 19 is an enlarged cross-sectional view of an essential portion ofanother modification of the fifth embodiment according to the presentinvention;

FIG. 20 is an enlarged cross-sectional view of the first and secondannular projections of another embodiment according to the presentinvention;

FIG. 21 is a cross-sectional view of a conventional viscous fluidcoupling; and

FIG. 22 is an enlarged cross-sectional view of a portion of FIG. 21.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 to 9, there is shown a first embodiment of aviscous fluid coupling 1 according to the present invention. The viscousfluid coupling 1 is adapted to drive a cooling fan (not shown) of aninternal combustion engine E. The viscous fluid coupling 1 comprises ahousing 4 which is rotatably supported to a drive shaft 2 through abearing 3, an operation plate 8 for controlling a fluid communicationbetween a reservoir 5 and an operation chamber 6 defined in the housing4, a rotor 9 installed in the operation chamber 6 and fixedly connectedto the drive shaft 2, a torque transmitting section 10 and a valvemechanism 11 for controlling a flow rate of viscous fluid flowing fromthe reservoir 5 to the operation chamber 6.

The torque transmitting section 10 includes a plurality of first annularprojections 13 concentrically formed on an outer periphery portion of adriven wheel 12 facing with the rotor 9 and a plurality of secondannular projections 14 formed on the rotor so as to be overlappedly orengagedly adjacent to the first annular projections 13 of the drivenwheel 12. The torque transmitting section 10 is arranged so that a partof the torque transmitting section 10 is located inside of an innersurface 5 a of the reservoir 5 with respect to a center axis C of theviscous fluid coupling 1. The viscous fluid is sealingly stored in thereservoir 5 and the operation chamber 6, and the torque transmittingoperation at the torque transmitting section 10 is executed through theviscous fluid.

The housing 4 includes a housing body member 21 which is rotatablysupported by the drive shaft 2 and a cover member 23 which is installedon a front portion of the housing body member 21 by means of bolts 22.The cover member 23 includes a circular recess portion 24 for definingthe reservoir 5 and a ring-shaped flange portion 26 having a returnpassage 25 for returning (draining) viscous fluid from the operationchamber 6. The flange portion 26 is formed at a peripheral side withrespect to the circular recess portion 24. The operation plate 8 isinstalled to the flange portion 26 so as to close the recess portion 24.

As shown in FIG. 3, the operation plate 8 is made of metal and is formedinto a generally disc shape. The first and second holes 7 and 7A areformed on the operation plate 8 so as to fluidly communicate thereservoir 5 and the operation chamber 6. As shown in FIG. 2, the outerperipheral portion of the operation plate 8 is fixedly connected to theflange portion 26 of the cover member 23 through four caulking portions27 by means of caulking so as to close an opening of the recess portion24 of the cover member 23. The four caulking portions 27 are formed onthe flange portion 26 at equal intervals.

A first surface of the operation plate 8 is faced with the reservoir 5and is partially covered with a valve plate 51 of a valve mechanism 11.The valve plate 51 is arranged to open and close the first and secondholes 7 and 7A of the operation plate 8. A second surface of theoperation plate 8 is faced with the operation chamber 6 and is connectedto a driven wheel 12. More specifically, the driven wheel 12 isoverlapped on the second surface of the operation plate 8 and is fixedto the cover member 23 by means of a plurality of bolts 28.

As shown in FIG. 4, formed on a surface of the driven wheel 12 coveringthe operation plate 8 are a recess portion 31 for engaging the caulkingportion 27, first and second sector-shaped auxiliary chambers 32 and 32Acommunicated with the first and second holes 7 and 7A respectively, andfirst and second passages 33 and 34 communicating the first and secondauxiliary chambers 32 and 32A with the torque transmitting portion 10.

The recess portion 31 is formed circular in shape. When the driven wheel12 is set on the operation plate 8 during assembly steps, the caulkingconnecting portion 27 are set in the recess portion 31 so that thesecond surface of the operation plate 8 is fitted with the driven wheel12. The driven wheel 12 is set on the operation plate 8 so that thefirst and second auxiliary chambers 32 and 32A are located at thepositions of the first and second holes 7 and 7A. The first and secondauxiliary chambers 32 and 32A are formed such that cross-sectional areasof them are greater than those of the first and second holes 7 and 7A.This arrangement functions to smoothly flow the viscous fluid from theholes 7 and 7A to the torque transmitting section 10. The first passage33 includes a first passage portion 33A through which the viscous fluidflows from the first hole 7 to the outer peripheral portion of thedriven wheel 12, and a first passage portion 33B formed at a tip endportion of the first passage portion 33A. The second passage 34 includesa second passage portion 34A through which the viscous fluid flows fromthe first hole 7 to the outer peripheral portion of the driven wheel 12,and a second passage portion 34B formed at a tip end portion of thesecond passage portion 34A.

As shown in FIG. 4, a distance L1 between a center of the driven wheel12 and the first passage 33B is different from a distance L2 between thecenter of the driven wheel 12 and the second passage portion 34B.Therefore, the timing that the viscous fluid is supplied to the torquetransmitting section 10 through the first passage 33 is different fromthe timing that the viscous fluid is supplied to the torque transmittingsection 10 through the second passage 34. This difference generatesvariations as to the operation characteristics.

As shown in FIGS. 5 and 6, the first annular projections 13 areconcentrically formed on the other surface of the driven wheel 12. Thefirst annular projections 13 constitute a part of the torquetransmitting section 10. The first annular projections 13 are set at thesame height H1.

FIG. 7 shows a modification of the driven wheel 12. In thismodification, four arc-shaped recess portions 31A for receiving thecaulking connecting portions 27 are formed at positions corresponding tothe four caulking connecting portions 27. This modified structure of therecess portions 31 decreases a volume of a space of the recess portions31A as compared with the annular recess portion 31 and thereforesuppresses the increase of the total volume of the viscous fluid.

As shown in FIGS. 8 and 9, the rotor 9 includes a bearing hole 9 a atthe center portion thereof. A tip end portion of the drive shaft 2 isinserted to the bearing hole 9 a and is fixed to the drive shaft 2 byenlarging a peripheral thereof. The second annular projections 14Aconstituting a part of the torque transmitting section 10 areconcentrically formed on a surface of the driven wheel 12 faced with therotor 9. The second annular projections 14A are set at the same heightH2, and the height H2 of the second annular projections 14A is generallythe same as the height H1 of the first annular projections 13. The rotor9 has a plurality of lightening or weight reducing holes 41 around thebearing hole 9 a as shown in FIG. 8.

As shown in FIG. 1, the valve mechanism 11 is fittingly set on theoperation plate 8. The valve mechanism 11 includes a valve plate 51 foropening and closing the first and second holes 7 and 7A and a coiledbimetal 52 for driving the valve plate 51 according to the temperaturethereof. The bimetal 52 is installed at a center portion of the covermember 23 and is connected to the valve plate 51 through a rotationshaft 53. Therefore, when ambient temperature around the housing 4 andthe bimetal 52 rises as high as a predetermined temperature, the bimetal52 extends to rotate the valve plate 51 in a direction through therotation shaft 52. By this rotation of the valve plate 51, the first andsecond holes 7 and 7A are sequentially opened. On the other hand, whenambient temperature around the housing 4 and the bimetal 52 falls to apredetermined temperature, the bimetal 52 is compressed to rotate thevalve plate 51 in the other direction. By the reverse rotation of thevalve plate 51, the first and second holes 7 and 7A are sequentiallyclosed.

Next, the manner of operation of the viscous fluid coupling will bediscussed.

When the engine is stopped, the viscous fluid is stored in the reservoir5, the operation chamber 6 and a fluid storing space formed behind therotor 9 become the same. When the engine E is started, the drive shaft 2and the rotor 9 are rotated by a crankshaft of the engine E. When theambient temperature around the housing 4 is lower than the predeterminedtemperature, the valve mechanism 11 is set at a close position whereinthe first and second holes 7 and 7A of the operation plate 8 are closedby the valve plate 51 so as to stop the circulation of the viscousfluid. Therefore, a flow rate supplied to the torque transmittingsection 10 is decreased and therefore the torque transmission from therotor 9 to the housing 4 is decreased so as to prevent a cooling fanfrom rotating.

When the ambient temperature around the housing 4 rises as high as thepredetermined temperature, the first and second holes 7 and 7A areopened by the opening operation of the valve mechanism 11. Therefore,the viscous fluid flows from the reservoir 5 to the operation chamber 6and the torque transmitting section 10, and therefore the torquetransmission from the rotor 9 to the housing 4 is established so as torotate the cooling fan at high speed. This high-speed rotation of thecooling fan sufficiently cools a radiator of the engine E.

Referring to FIG. 10, there is shown a second embodiment of the viscousfluid coupling 1 according to the present invention. The secondembodiment is generally the same as the first embodiment except that theheight H2 of the second annular projections 14B of the rotor 9 isshorter than that of the second annular projections 14A of the firstembodiment. More specifically, the height H2 of the second annularprojections 14B of the second embodiment is set from one-half totwo-third of the height H1 of the first annular projections 13 of thedriven wheel 12. The manner of operation of the viscous fluid coupling 1of the second embodiment is basically the same as that of the firstembodiment. Therefore, the explanation thereof is omitted herein. Inaddition to the advantages of the first embodiment, the secondembodiment provides a further advantage. That is, since the secondembodiment of the viscous fluid coupling 1 according to the presentinvention is arranged such that the height H2 of the second annularprojections 14B is set from one-half to two-third of the height H1 ofthe first annular projections 13 of the driven wheel 12, the viscousfluid is further effectively discharged from the torque transmittingsection 10 while suppressing the torque transmission quantity of thetorque transmitting section 10. This further suppresses the generationof a dragging rotation of the cooling fan at the start of the engine Eand decreases the time for the dragging rotation of the cooling fan.Further, an overshoot rotation during the engine accelerating state isreduced.

Referring to FIGS. 11 and 12, there is shown a third embodiment of theviscous fluid coupling 1 according to the present invention. In thisthird embodiment, the viscous fluid coupling 1 is arranged such that thetorque transmitting section 10 is formed at a height or level which ishigher than a fluid level L of the viscous fluid 61 which remains in thereservoir 5 when the engine E is stopped. Further, the lightening holes41 are through-holes penetrating the rotor 9 in the thickness direction.The lightening holes 41 are formed to be located at a height levelhigher than the liquid lever L of the viscous fluid 61 during enginestoppage. This arrangement of the lightening holes 41 functions toprevent the viscous fluid 61 from flowing from the liquid storage space52 to the torque transmitting section 10 though the lightening holes 41.In this embodiment the annular projections are denoted by 14C. The otherconstruction of the viscous fluid coupling 1 of the third embodiment isthe same as that of the first embodiment. Therefore the explanationthereof is omitted herein.

Although the third embodiment according to the present invention hasbeen shown and described such that the lightening holes 41 are formed topenetrate the rotor 9, it will be understood that the lightening holes41 may be arranged to be blind and not to completely penetrate the rotor9. If the lightening holes 41 are arranged to be blind and not tocompletely penetrate the rotor 9, the lightening holes 41 may be locatedat a position lower than the liquid level.

Referring to FIGS. 13 and 14, there is shown a fourth embodiment of theviscous fluid coupling 1 according to the present invention. Theconstruction of the fourth embodiment is generally the same as that ofthe third embodiment except that the height of the second annularprojections 14D of the rotor 9 is arranged such that the height of thediametrical or radially inner-side second annular projections 14D is setas high as the height of the first annular projections 13, and theheight H3 of the diametrically or radially outer-side second annularprojections 14D is set from one-half to two-third of the height H1. Morespecifically, the overlapped portion between the first annularprojections 13 and the second annular projections 14D is varied suchthat an overlapped amount at the diametrical outer side of theoverlapped portion is set from one-half to two-third of the overlappedamount at the inner side. With this arrangement of the fourthembodiment, the discharging of the viscous fluid from the outerperipheral portion of the torque transmitting section 10 is facilitatedand quickly executed. Therefore, even when the outer peripheral portionof the torque transmitting section 10 is dipped in the viscous fluid bythe stopping of the engine E, the remaining viscous fluid in the outerperipheral portion of the torque transmitting section 10 is easilydischarged by starting the engine E. This suppresses the draggingrotation of the cooling fan. Since the other construction of the fourthembodiment is the same as that of the first embodiment, the explanationthereof is omitted herein.

Referring to FIGS. 15 and 16, there is shown a fifth embodiment of theviscous fluid coupling 1 according to the present invention. Theconstruction of this fifth embodiment is also generally the same as thatof the first embodiment except that the height H2 of the second annularprojections 14E are different from those of the first embodiment. Morespecifically, the second annular projections 14E formed on the rotor 9are arranged such that the height H2 of the second annular projections14E are gradually decreased from the inner side to the outer side in thediametrical direction as shown in FIG. 16. The height H2 of theinnermost second annular projection 14E is generally the same as theheight H1 of the first annular projections 13. The height H2 of theoutermost second annular projection 14E is set from one-half totwo-third of the height H1. The height H1 of the first annularprojections 13 of this fifth embodiment are constant from the inner sideto the outer side in the diametrical direction as same as that of thefirst embodiment.

That is, the overlapped amount between the first annular projection 13and the second annular projections 14E are gradually decreased frominside to outside in the diametrical direction. Therefore, the viscousfluid discharging function is improved toward the outer peripheral side.The other construction of the fifth embodiment is the same as that ofthe first embodiment, and therefore the explanation thereof is omittedherein.

Although the first to fifth embodiments have been shown and describedsuch that the first annular projections 13 are formed on the drivenwheel 12, it will be understood that the first annular projections 13may be formed on the cover member 23 or the housing body member 21 asshown in the sixth to eighth embodiments shown in FIGS. 17 to 19.Furthermore, it will be understood that the first and second annularprojections 13 and 14(F-H) may be respectively arranged such that bothof the height H1 of the first annular projections 13 and the height H2of the second annular projections 14(F-H) are gradually decreased frominside to outside in the diametrical direction as shown in FIG. 20. Thisarrangement also decreases the overlapped amount between the first andsecond annular projections 13 and 14F toward the outer peripheral side.

What is claimed is:
 1. A viscous fluid coupling comprising: a driveshaft adapted for connection to a rotation shaft of an engine; a housingrotatably supported on the drive shaft; an operation plate set in thehousing and dividing a space defined within the housing into a viscousfluid reservoir and an operation chamber, the operation plate comprisinga communication hole communicating the reservoir and the operationchamber; a rotor disposed in the operation chamber and fixed to thedrive shaft; a valve mechanism opening and closing the communicationhole according to a temperature of the housing to control a flow rate ofa viscous fluid from the viscous fluid reservoir to the operationchamber; and a torque transmitting section comprising a plurality offirst circumferential projections concentrically formed on a portion ofthe housing and a plurality of second circumferential projectionsconcentrically formed on an outer peripheral portion of the rotor; thefirst circumferential projections being interleaved with the secondcircumferential projections and being adapted to be fluidly coupled witheach other through the viscous fluid, wherein: an overlap amount of aradially outer couple of an outermost first circumferential projectionand an outermost second circumferential projection is smaller than anoverlap amount at a radially inner couple of an innermost firstcircumferential projection and an second innermost circumferentialprojection, and wherein: the housing is dimensioned such that when theengine is stopped, the housing is stationary, and the housing contains apredetermined amount of viscous fluid, a level of the viscous fluid inthe operation chamber being such that the outermost first and secondcircumferential projections on the housing and the rotor respectively,are dipped in the viscous fluid.
 2. A viscous fluid coupling as claimedin claim 1, wherein the engine is a vehicular internal combustion engineand wherein the viscous fluid coupling is operatively connected to acooling fan of the vehicular internal combustion engine.
 3. A viscousfluid coupling, comprising: a drive shaft adapted for connection to arotation shaft of an engine; a housing rotatably supported to the driveshaft; a predetermined amount of viscous fluid disposed in the housing;an operation plate set in the housing and dividing a space definedwithin the housing into a viscous fluid reservoir and an operationchamber into which the viscous fluid can be introduced from thereservoir, the operation plate comprising a communication holecommunicating the reservoir and the operation chamber to permit viscousfluid to transfer between the reservoir and the operation chamber; arotor disposed in the operation chamber and fixed to the drive shaft; avalve mechanism opening and closing the communication hole according toa temperature of the housing to control a flow of a viscous fluid fromthe viscous fluid reservoir to the operation chamber; and a torquetransmitting section comprising a plurality of first circumferentialprojections concentrically formed on a portion of the housing and aplurality of second circumferential projections concentrically formed onan outer peripheral portion of the rotor, the first circumferentialprojections being spacedly interleaved with the second circumferentialprojections and adapted to be fluidly coupled with each other throughthe viscous fluid, an axial dimension of a radially inner projection ofthe circumferential projections of at least one of the housing and therotor being greater than an axial dimension of a radially outerprojection of the circumferential projections of the at least one of thehousing and the rotor, and wherein the housing is dimensioned such thatwhen the engine is stopped and the housing is stationary, thepredetermined amount of viscous fluid in the operating chamber is suchthat a radially outermost circumferential projection on the housingportion and a radially outermost circumferential projection on the rotorare dipped in the viscous fluid.
 4. A viscous fluid coupling,comprising: a drive shaft adapted for connection to a rotation shaft ofan engine; a housing rotatably supported to the drive shaft, apredetermined amount of viscous fluid being disposed in the housing; anoperation plate set in the housing and dividing a space defined withinthe housing into a viscous fluid reservoir and an operation chamber, theoperation plate comprising a communication hole communicating viscousfluid between the reservoir and the operation chamber; a rotor disposedin the operation chamber and fixed to the drive shaft; a valve mechanismopening and closing the communication hole according to a temperature ofthe housing to control flow of the viscous fluid from the viscous fluidreservoir to the operation chamber; and a torque transmitting sectioncomprising a plurality of first circumferential projectionsconcentrically formed on the housing and a plurality of secondcircumferential projections concentrically formed on the rotor; wherein:the first circumferential projections are interleaved with the secondcircumferential projections so as to overlap therewith and are adaptedto be fluidly coupled with each other through the viscous fluid, anaxial dimension of a radially inner projection of the circumferentialprojections in a direction parallel to an axis about which the rotor isrotatable, on one of the housing and the rotor, is greater than an axialdimension of a radially outer projection in a direction parallel to anaxis about which the rotor is rotatable, of the circumferentialprojections of the other of the housing and the rotor, thecircumferential projections of the other of the housing and the rotorhaving the same axial dimension, and wherein the housing is dimensionedsuch that when the housing is stationary, a level of the predeterminedamount of viscous fluid in the operating chamber is such that of aradially outermost circumferential projection on the housing and aradially outermost circumferential projection the rotor are dipped inthe viscous fluid.
 5. A viscous fluid coupling, comprising: a driveshaft adapted for connection to a rotation shaft of an engine; a housingrotatably supported to the drive shaft; an operation plate set in thehousing and dividing a space defined within the housing into a viscousfluid reservoir and an operation chamber, the operation plate comprisinga communication hole communicating the viscous fluid reservoir and theoperation chamber; a rotor disposed in the operation chamber and fixedto the drive shaft; a valve mechanism controlling the communication holeaccording to a temperature of the housing to control a flow of a viscousfluid from the viscous fluid reservoir to the operation chamber; and atorque transmitting section comprising a plurality of firstcircumferential projections concentrically formed on the housing and aplurality of second circumferential projections concentrically formed onthe rotor, the first circumferential projections being interleaved withthe second circumferential projections and adapted to be fluidly coupledwith each other through the viscous fluid, wherein: the firstcircumferential projections each having the same axial dimension, takenin a direction parallel to an axis of rotation of the rotor, an axialdimension of a radially inner one of the second circumferentialprojections, taken in a direction parallel to the axis of rotation ofthe rotor, is greater than an axial dimension of a radially outer one ofthe second circumferential projections, and wherein: the housing is soshaped and dimensioned such that when the housing is stationary, a levelof the viscous fluid in the operating chamber is such that a radiallyoutermost circumferential projection on the housing portion and aradially outermost circumferential projection of the rotor are dipped inthe viscous fluid.
 6. A viscous fluid coupling connected to an engine,comprising: a drive shaft connected to a rotation shaft of the engine; ahousing rotatably supported on the drive shaft; an operation plate setin the housing and dividing a space defined by the housing into areservoir and an operation chamber, the operation plate comprising acommunication hole communicating the reservoir and the operationchamber; viscous fluid in the reservoir and the operation chamber; arotor disposed in the operation chamber and fixed to the drive shaft; avalve mechanism opening and closing on the communication hole accordingto a temperature of the housing to control a flow rate of the viscousfluid from the reservoir to the operation chamber; and a torquetransmitting section comprising a plurality of first circumferentialprojections concentrically formed on an outer peripheral portion of thehousing and a plurality of second circumferential projectionsconcentrically formed on an outer peripheral portion of the rotor,wherein: the first circumferential projections are interleaved with thesecond circumferential projections and adapted to be fluidly coupledwith the second circumferential projections through the viscous fluid,an axial dimension of a radially outer projection of the circumferentialprojections of one of the housing and the rotor is smaller than an axialdimension of a radially inner projection of the circumferentialprojections of the one of the housing and the rotor, the circumferentialprojections of the one of the housing and the rotor have two kinds ofaxial dimensions, and the outer projections of the housing and the rotorbeing dipped in the viscous fluid when the engine is in an inoperativestate.
 7. The viscous fluid coupling as claimed in claim 6, wherein theaxial dimension of the radially outer projection of the circumferentialprojections of the one of the housing and the rotor ranges fromtwo-third to a half of the axial dimension of the radially innerprojection of the circumferential projections of the one of the housingand the rotor.
 8. The viscous fluid coupling as claimed in claim 7,wherein the axial dimension of the radially inner projection of thecircumferential projections of the one of the housing and the rotor isequal to an axial dimension of the circumferential projections of theother of the housing and the rotor.
 9. The viscous fluid coupling asclaimed in claim 6, wherein an overlap amount of an outermost firstcircumferential projection and an outermost second circumferentialprojection ranges from two-third to a half of an overlap amount of aninnermost first circumferential projection and an innermost secondcircumferential projection.
 10. The viscous fluid coupling as claimed inclaim 6, further comprising a groove extending in the radial directionand formed on the circumferential projections of the one of the housingand the rotor.
 11. The viscous fluid coupling as claimed in claim 6,wherein the engine is a vehicular internal combustion engine and whereinthe viscous fluid coupling is operatively connected to a cooling fan ofthe vehicular internal combustion engine.
 12. A viscous fluid couplingconnected to an engine, comprising: a drive shaft connected to arotation shaft of the engine; a housing rotatably supported on the driveshaft; an operation plate set in the housing and dividing a spacedefined by the housing into a reservoir and an operation chamber, theoperation plate comprising a communication hole communicating thereservoir and the operation chamber; viscous fluid in the reservoir andthe operation chamber; a rotor disposed in the operation chamber andfixed to the drive shaft; a valve mechanism opening and closing on thecommunication hole according to a temperature of the housing to controla flow rate of the viscous fluid from the reservoir to the operationchamber; a torque transmitting section comprising a plurality of firstcircumferential projections concentrically formed on an outer peripheralportion of the housing and a plurality of second circumferentialprojections concentrically formed on an outer peripheral portion of therotor, the first circumferential projections being interleaved with thesecond circumferential projections and adapted to be fluidly coupledwith the second circumferential projections through the viscous fluid, aradially outer projection of the circumferential projections of one ofthe housing and the rotor being dipped in the viscous fluid when theengine is in an inoperative state, an axial dimension of the radiallyouter projection of the circumferential projections of the one of thehousing and the rotor is smaller than an axial dimension of a radiallyinner projection of the circumferential projections of the one of thehousing and the rotor, and the circumferential projections of the one ofthe housing and the rotor having two kinds of axial dimensions.
 13. Theviscous fluid coupling as claimed in claim 12, wherein the axialdimension of the radially outer projection of the circumferentialprojections of the one of the housing and the rotor ranges fromtwo-third to a half of the axial dimension of the radially innerprojection of the circumferential projections of the one of the housingand the rotor.
 14. The viscous fluid coupling as claimed in claim 13,wherein the axial dimension of the radially inner projection of thecircumferential projections of the one of the housing and the rotor isequal to an axial dimension of the circumferential projections of theother of the housing and the rotor.
 15. The viscous fluid coupling asclaimed in claim 12, wherein an overlap amount of an outermost firstcircumferential projection and an outermost second circumferentialprojection ranges from two-third to a half of an overlap amount of aninnermost first circumferential projection and an innermost secondcircumferential projection.
 16. The viscous fluid coupling as claimed inclaim 12, further comprising a radially extending groove formed on thecircumferential projections of the one of the housing and the rotor. 17.The viscous fluid coupling as claimed in claim 12, wherein the engine isa vehicular internal combustion engine and the viscous fluid coupling isoperative connected to a cooling fan of the vehicular internalcombustion engine.